Cross-linked isotactic polybutene-1

ABSTRACT

Isotactic polybutene-1 is cross-linked with sulfur and an organic peroxide which decomposes at 140-220*C, e.g., dicumylperoxide.

United States Patent Seifert et a1. June 6, 1972 1 541 CROSS-LINKED ISOTACTIC POLYBUTENE-l [56] References Cited [72] Inventors: Friederich Seifert, Marl; Josef Bittscheidt, UNITED STATES PATENTS Datteln', Johannes Plenikowski, Marl, all of German 3,159,596 12/1964 Falcone ..260/41 Y r 3,329,647 7/1967 Sermuk ..260/41 [73] Assignee: Chemische Werke Huls A.G., Marl, Ger- 3,329,649 7/ 1967 Wei 1 .260/41 3,406,732 10/1968 Milano. 152/330 [22] Filed: 12, 1970 3,477,985 11/1969 Bucci ..260/41 [21 App]. No.: 26,464 FOREIGN PATENTS OR APPLICATIONS Related us. Application Dam 715,153 8/1965 Canada ..260/94.9

[62] Division of Ser. No. 553,014, May 26, 1966, Pat. No. P i Examiner-Joseph L. Schofer Assistant Examiner-C. A. Henderson, Jr.

' Attorney-l. William Millen, M. Ted Raptes and John L. [30] Foreign Application Priority Data whi May 28, 1965 Germany ..C 35 9791 ABSTRACT U.S. P, lsotactic polybutene l is cross ]inked ulfur and an or- A ganic peroxide which decomposes at 140220C, e.g., dicu- [51] Int. Cl. ..C08f 27/00 1 [58] Field of Search... .....260/79.5 P, 79.5 GA, 94.9,

260/775 9 Claims, No Drawings crystalline/synthetic ,resins, and more particularly to 'a novel process of initiatingcross-linking of polybutene-l during the extrusion thereof.

It has been conventional heretofore to effect cross-linking of polyethylene andthe higher homologs thereof, i.e., isotactic polypropylene, .andpoly-a-butene, with the aid of peroxides (German Published applications No. 1,187,789 and 1,186,210). When these higher homologs are cross-linked, however, it should be accomplished with a mixture of aromatic, polymerizablehydrocarbon monomers containing a predominant amount of divinyl benzene, in combination with saturated arylalkyl hydrocarbons.

Such cross-linkable polymers can be compression-molded with case, but due to their high viscosity they cannot be easily shaped by conventional continuous thermoplastic processing techniques, such as the extrusion of tubes or cable jackets. To overcome the difficulties in extruding crossJinkable polyethylene, it has been proposed to provide special features, such as a high-frequency field about the extruder head. However, since polyethylene is an apolar synthetic resin, it remains unaffected by the alternating field, it being therefore necessary to employ substantial amounts of carbon black in the polymer, Consequently, hydrocarbon polymers which are not loaded with conductive fillerscannot be extruded satisfactorily by this special method.

In German Patent No. 953,744, it is disclosed that polyisobutylene can be hardened with sulfur and tert.-butyl peroxide, if desired in the presence of an accelerator. Owing to the rubbery non-crystalline characteristics of the products, it would not be expected that they could be shaped by simple continuous thermoplastic processing techniques, since the general rule is that rubbery polymers must be processed by the methods developed in rubber technology. This expectation is universal where vulcanization of the polymer is conducted at high temperatures without any substantial internal fiow within the shaped mass.

' To produce a cross-linked polymer of polybutene-l, it has been disclosed that it is necessary to employ a peroxide crosslinking agent with, other monomers such as divinyl benzene (German Published application No. 1,186,210), inasmuch as peroxides per se do not work satisfactorily. Despite the crystallinity of the .polybutene-l, the vulcanizable mixture of the divinyl benzene ,and peroxide was vulcanizedin a heated compression mold. As a matter of interest, owing to the difficulties in continuously processing vulcanizable polyethylene, it was to be expected that similar processing difficulties would be experienced with any attempts to shape cross-linkable polybutene-l in a continuous manner.

It is, therefore, a principalobject of the present invention to provide animproved'method of continuously extrudinga vulcanizable mixture of polybutene- 1.

Another object of thisinvention is to provide anovel vulcanizable mixture of polybutene- 1.

Still another object of this invention is to provide'a novel device for use in the extrusion of ,cross-linkable, thermosetting synthetic resins.

These and other objects and advantages of the present invention will become apparent by reference to the following description, claims, and attached drawing.

To attain the objects of this invention, it has been unexpectedly found that isotacticpolybutene-l can be simultaneously cross-linked .and extruded'into any desired shape by incorporating therein an organic peroxide, preferably 0.005 to 2 percent by weight; sulfur, preferably 0.05 to 4 percent by weight; intheoptional presence of an accelerator, preferably 0.1 to ,2.percent byweight; and by heating=the resultant mass as it passes through the nozzle of the extruder to a temperature at whichthe peroxide decomposes and initiates crosslinking. Parts byweight are based on isotactic polybutene-l Isotactic polybutene-l is'particularly suitable'for use herein and desirably has an average molecular weight of 500,000 to 5,000,000, preferably 1,000,000 to 3,000,000; these molecular weights correspond to a specific reduced viscosity n,,,,,) of about 1 to 10, preferably 2 to 6. These isotactic polymers can be obtained according to conventional methods of stereospecific polymerization, such asdescribed in "Linear :Interscience Publishers, Inc, NY. 1959), and elsewhere.

Preferably, suitable isotactic polybutene-l materials are produced by the Ziegler-Nattznprocess.

The organic peroxides of this invention must be capable of reacting with the polymericchains as they are passed through the nozzle. Preferably, the peroxides must be reactive at temperaturesbelow 250C more preferably in the range of 130 C. to 220 C. Such peroxides can be symmetrical or asymmetrical, and can be substituted by aliphatic, aromatic, or cycloaliphatic groups, preferably hydrocarbon groups of one to 10 seven to 12 and six to 10 carbon atoms, respectively. Examples of specific peroxides are dicumyl peroxide, dibenzoyl peroxide, tert.-butyl perbenzoate, tert.-butyl-acumyl peroxide, di-tert.-butyl peroxide, dibenzyl peroxide, bis-(tert.-butyl-peroxymethyl)-durene, 2-,5-dimethyl-2-,5- peroxy-3-hexyne (containing a triple bond)2-,5-dimethyl-2- ,5-diisopropylperoxy-hexane, diisopropylperoxy-hexane, 2,2- bis-(tert.-butyl-peroxy)-butane, or isoprop'ylidene2,5- dimethylhexane-2,S-diperoxide. Although the above-mentioned peroxides are preferred, other organic peroxides can be used; the criterion is that they remain percent stable at temperatures up to at least C., and decompose when heated to about to 220 C.

The sulfur to be used can be in any crystalline state, although it is preferred to employ flowers of sulfur (refined by sublimation).

Suitable accelerators which can be used are, for example, diphenyl guanidine, di-o-toluyl guanidine, mercaptobenzothiazole, tetramethyl thiuram-monoor disulfide,

crotone vinylidenetetramine, zinc benzothiazole mercaptide, or zinc oxide with stearic acid.

In preferred embodiments of this invention, there are employed as additives to the polybutene-l the following combinations of materials:

a. 0.05 to 1 percent by weight of a peroxide selected from the group consisting of 2-,5-dimethyl-2,5-peroxy-3-hexine, 2- ,5-dimethyl-2,S-diisopropylperoxy-hexyne, isopropylidene- 2,S-dimethyl-hexane-Z,5-diperoxide, and mixtures thereof;

b. 0.2'to 0.5 percent by weight of sulfur; and optionally c. 0.3 to 0.8 percent'by weight of an accelerator selected from the group consisting of mercaptobenzothiazole, tetramethyl thiuram-disulfide, and mixtures thereof, or

d. l to 2'percent by weight of zinc oxide with 0.1 to 0.5 percent by weight of stearic acid, and/or e. zinc benzothiazole mercaptide are added for even better results.

The particular amount of these additives to be used in any case is dependent upon the molecular weight-of the polybu tene-l which is employed.

In addition to the above-described additives, there can also be incorporated conventional antioxidants, such as ionol (2,6- di-tert.-butyl-4-methyl-phenol ),4,4"thiobis-( 6-tert.-butyl-3 methyl-phenol) or N-stearoyl-p-aminophenol (see also Kirk- Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 2, pp. 599--601 mold release agents, such as calcium stearate, or also sodium stearate (0.5 to 0.3 percent); pigments, such as titanium dioxide, cadmium sulfide, carbon black; and fillers, for example 10 to 20 percent of chalk.

It hasbeen found tobe particularly advantageous to incorporate in the polymer 20 to 60, preferably 23 to 40 percent by weight of carbon'black,such as channel black, furnace black,

or furnace thermal black. The use of carbon black not only lowers the surface resistance of the cross-linked finished products from 10 to values below 10 .0, but also effects permanent destaticization of these products.

The several aforementioned additives can be admixed with the polybutene-l individually in any order, or in any concentration. It is, however, necessary to maintain the temperature of the polymer during mixing below the decomposition temperature of the peroxide employed, usually not above 120 C. when agglomerating, 160 C. when extruding the mixture. Should the temperature of the polymer during the mixing step be above the decomposition temperature of the peroxide, it is then desirable to add the peroxide during the last phase of the subsequent granulation process.

In practice, the various additives are incorporated into particles of the polybutene-l having a diameter of 40 to 600 a. preferably 100 to 500 ,u.. Since blockage ofthe screw in the extruder can occur when using fine-mesh particles, it is therefore desirable first to increase the size of these particles by agglomerating the same. This process can be carried out in a heated mixing vessel; sintering takes place at temperatures between 1 10 C and 120 C, resulting in particles ofa diameter 01 100 to 1000 a.

Instead of this process it is possible to produce a granulate by extruding the mixture at temperatures between 140 and 160 C., cutting the so formed worm into particles having an average particle size of2 to mm.

After passing one of these processes, there is obtained a vulcanizable polymer of polybutene-l in a form that may be loaded into a feed hopper, whence the particles fall into the cylinder of the extruder to provide a continuous feed. As the particles move forward in the thread of the screw, the temperature thereof rises due to the heated walls of the cylinder and also on account of the friction developed within the polymer as a result of the combination of the rotation and the compression due to the packing action of the screw. Thus, as the particles of polybutene-l advance through the cylinder toward the die, they become transformed from a mass of granules into a continuous homogeneous plasticized mass which is readily forced through the orifice to accept the desired shape.

Although we do not intended to bound by an explanation of the cross-linking, it is believed that the peroxides, in combination with the sulfur, acts as a free radical donor to polybutene- 1 chains by removing a hydrogen and providing a free radical in the chain. These free radicals are then capable ofcross-linking polybutene-l chains. Since even peroxides decomposing at high temperatures, such as 2-,5dimethyl-2-,5-diisopropylperoxy-hexane, or isopropylidene-2,5-dimethyl-hexane-2,5- diperoxide, decompose at elevated temperatures and can initiate premature cross-linking, it is preferred to maintain the temperature of the polymer mass as low as feasible within the boundaries of good extrusion conditions as it is moved forward by the screw in the plasticizing zone of the extruder. Since the peroxide begins to decompose even at 160180 C., it is particularly desirable to minimize the residence time of the polymer at such temperatures and higher as it passes through the screw section, preferably on the order of 3 to 5 minutes. The residence time in the nozzle is generally on the order of 1.5 to 3 minutes.

As the compressed and plasticized polymer emerges from the compression zone of the extruder, it is passed into the nozzle section wherein it is further heated to a temperature at which the peroxide decomposes and initiates the complete cross-linking of the polymer. Although the initiation of crosslinking can depend, in part, upon the particular machine employed, e.g., the construction of the screw, the length of the screw, compression ratio, shape of nozzle, and rotational speed of the screw, it has been found that the desired crosslinking can be realized by continuously elevating the temperature of the mass from the plasticizing zone (l30180C.) to the nozzle to about 180-240 C: the temperature of the polybutene-l is, on the average. about to 30 higher than those temperatures otherwise employed when working polybutenel.

In another embodiment of this invention, polybutene-l in admixture with the aforementioned additives can be readily shaped in discontinuous processes, such as in compression molds. In such cases, the granulated polymer mixture is charged into the mold and heated for about 1-2 minutes at 180-190 C., whereupon the shaped article can be removed from the mold without the danger of deformation after being cooled.

The favorable properties of the novel molded masses are particularly surprising because polybutene-I is subjected to chain scission by peroxide by itself, but not cross-linked, and because sulfur, even in the presence of accelerators, likewise does not effect cross-linking. This is in contradistinction to, for example, the effectiveness of these substances in polyethylene or in ethylene-propylene rubber wherein the peroxide by itself has a cross-linking effect. In such cases sulfur is not required but serves merely as an additive which improves certain properties, such as the durability of the crosslinkages or the time for reaching the final degree of vulcanization while at the same time sulfur deleteriously affects other properties, such as heat-age resistances and compression set.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the specification and claims in any way whatsoever.

EXAMPLE 1 In this example, the compositions of polybutene-l designated below as mixtures I, la and lb, and wherein 1 contains neither sulfur nor peroxide (only carbon black and antioxidants); la contains an addition ofa sulfur, accelerator and activator but no peroxide; and lb contains an addition of peroxide but without the sulfur system of In. A mixture of polybutene-l containing both the sulfur system of la and the peroxide of lb is designated below as mixture 11 and was also prepared to illustrate the advantages of the compositions of the present invention. Mixturel parts by weight of polybutene-l (n 4.5)

1.5 parts by weight of thermal black 0.1 parts by weight of 4,4'-thiobis-(3-methyl-6-tert.-bu-

tylphenol) 0.1 parts by weight ofdilauryl thiodipropionate Mixture la I Corresponds to mixture 1; additionally, it contains 0.3 parts by weight ofsulfur 0.3 parts by weight oftetramethyl thiuras-disulfide 0.15 parts by weight of mercaptobenzothiazole 0.2 parts by weight of stearic acid 1.5 parts by weight ofzinc oxide Mixture lb Corresponds to mixture 1; additionally, it contains 0.5 parts by weight of 2,5dimethyl-2-,S-peroxy-hexane Mixture 11:

100 parts by weight of polybutene-l (1;, ,,-4.5)

1.5 parts by weight ofthermal black 0.1 parts by weight of 4,4'-thiobis-( 6-tert.-butyl-m-cresol) 0.1 parts by weight of dilauryl thiodipropionate 0.5 parts by weight of 2-,5dimethyl-2,S-peroxy-hexanediisopropyl 1.5 parts by weight of zinc oxide 0.2 parts by weight of stearic acid 0.3 parts by weight of tetramethyl thiuram-disulfide 0.15 parts by weight of mercaptobenzothiazole 0.3 parts by weight ofsulfur Each of the mixtures is produced from the powdered materials in a high-speed mixer at room temperature, and then, during the course of about 10 to 15 minutes, the temperature is either raised to about to C. for the purpose of producing an agglomerate, or the powdered mixture produced at room temperature is granulated between and C., as already described above.

Thereupon, the granulate is cooled and then charged into the feed hopper of an extruder having a 15 D Short Compression screw,.a compression ratio of l 3, and a speed of rotation of about 30 r.p.m. As the mixture at [90 C. passed from the screw ,section or plasticizing zone to the nozzle zone, the temperature of the mixture increases to 230C, the plastic mass passing through the die is formed into tubes having an outside diameter of 32 mm and a wall thickness of 3 mm. The extrusion of'mixture lb did not produce satisfactory tube material owing to extensive chain scission of the polymer (lowering of the 1 value below 2). Consequently, the permanent stability thereof could not be determined.

To determine the permanent stability of the tubes produced from mixtures I and II, they were subjected to a continuous stress at 90 C. and a 0'-value of 60 at 12 atmospheres gauge, this test being conducted analogously to DIN 8074 and din 8075 for polyethylene. The surface resistance values of the tubes were also determined according to DIN 53 482/VDE 0303, Part 3.

By comparing the results of these tests, as shown in Table I, it can be seen that the permanent stability of the tubes measured at a 1 value of 60, and 90 C., increases due to crosslinking, from an average of 4,300 hours to an average of 7,750 hours. 1

I Moreover, the cross-linked polybutene-l tubes produced from mixture ll exhibit only a slight softening at about above 200 C. in comparison to the molting range of the extruded non-cross-linked polymers of mixtures I, la and lb of l24l 30 The solvent stability of pressed plates produced from the mixtures designated in Table I was also determined by immersing the same-in boiling toluene over a period of 2 hours. Only about percent by weight of the plates produced from mixture 4 II are dissolved, while corresponding samples produced from mixtures I, la, and lb, under the same conditions, are almostcompletely dissolved.

In addition to the production of tubes as described above, the vulcanizable polymers of the present invention can be extruded or otherwise formed into any desired shape.

EXAMPLE 2 To further illustrate the utility of the cross-linking agents of the present invention, additional compositions of polybutenel designated as mixtures Ill and IV were prepared by increasing the amount of carbon black in the basic mixtures I and II described in Example 1, to parts by weight of a furnace black (98% C, 0.3% II, 0.1% N, 0.7% S, and 0.9% 0) per 100 parts by weight of polybutene-l. These mixtures are then extruded and the resultant tubing tested according to the procedure described in Example 1. It can be seen from Table I that the increased concentration of furnace black in mixtures Ill and IV yields extruded tubing which is permanently antistatic (surface resistance below 10" O). The measured permanent stability of the tubes produced from mixture III is significantly lower than the permanent stability of the crosslinked tube produced from mixture IV. As shown in Table I, it

can be seen that the pennanent stability increases due to cross-linking, from an average of 4.300 hours to an average of 7.800 hours.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

What is claimed is:

l. A vulcanizable polymeric mass comprising uncrosslinked isotactic polybutene-l and vulcanizing amounts of a vulcanizing agent consisting of sulfur and an organic peroxide which is reactive with saidpolybutene-l and sulfur at temperatures below 250 C., said uncross-linked isotactic polybutene-l being essentially uncross-linkable by said organic peroxide alone.

2. A polymeric mass as defined by claim 1 wherein there are employed 0.005 to 2 percent by weight of an organic peroxide, and 0.05 to 4 percent by weight of sulfur.

3. A polymeric mass as defined by claim 2, further comprising an accelerator selected from the group consisting of mercaptobenzothiazole, tetramethyl thiuram-disulfide, and mixtures thereof.

4 A polymeric mass as defined by claim I wherein there is incorporated from 20 to percent by weight of carbon black.

5. A polymeric mass as defined by claim 1 wherein said organic peroxide is dicumyl peroxide, dibenzoyl peroxide, tert.- butyl perbenzoate, tert.-butyl-a-cumyl peroxide, di-tert.-butyl peroxide, dibenzyl peroxide, bis-(tert-butyl-peroxy-methyl)- durene, 2-,5-dimethyl-2-,5-peroxy-3 -hexine, 2-,5-dimethyl-2- ,5-diisopropyl-peroxy-hexane, 2,2-bis-(tert.-butyl-peroxy)-butane, or isopropylidene-2,5-dimethylhexane-2,S-diperoxide.

6. A polymeric mass as defined by claim 2 wherein said organic peroxide is dicumyl peroxide, dibenzoyl peroxide, tert.- butyl perbenzoate, tert.-butyl-a-cumyl peroxide, di-tert.-butyl peroxide, dibenzyl peroxide, bis-(tert.-butyl peroxy-methyl)- durene, -peroxide, 2-,5-dimethyl-2-,5-peroxy-3-hexine, 2-,5- dimethyl-2-,Sdiisopropyl-peroxy-hexane, 2,2-bis-(tert.-butylperoxy)-butane, or isopropylidene-2,5-dimethylhexane-2,5- diperoxide.

7. A polymeric mass as defined by claim 3 wherein said organic peroxide is dicumyl peroxide, dibenzoyl peroxide, tert.- butyl perbenzoate, tert.-butyl-a-cumyl peroxide, di-tert.-butyl peroxide, dibenzyl peroxide, bis-(tert.-butylperoxy-methyl)- durene, peroxide, 2-,5-dimethyl2-,5-peroxy-3-hexine, 2- ,5dimethyl-2-,5-diisopropyl-peroxy-hexane, 2,2-bis-(tert. butyl-poroxy)-butane, or isopropylidene-Z,S-dimethylhexane- 2,5-diperoxide.

8. A polymeric mass as defined by claim 1, said organic peroxide being present in a concentration of 0.05-l percent by weight, and being selected from the group consisting of 2- ,5dimethyl-2, 5-peroxy-3-hexyne, 2-,5-dimethyl-2,5-diisopropylperoxy-hexane, isopropylidene-2,5-dimethyl-hexane-2,5- diperoxide, and mixtures thereof, and said sulfur being present in a concentration of 0.2-0.5 percent by weight.

TABLE I.-COMPARISON OF PERMANENT STABILITY VALUES OF TUBING MADE FROM CROSS-LINKED AND NOT CROSS- LINKED POLYBUTENE-l Test at 12 atmospheres gauge, 90 0., v-value 60 Mixture I Ia Ib II III IV Permanent stability (hours) at a 60 C and 00 C 4,100-4,500 moo-4,500 Omitted 7,eo0-7,900 4,100-4,500 7,0008,000 10 10 1o 10 10 10 Surtace resistance in 0 according to DIN 53462/VDE 0303, part 3.

9. A polymeric mass as defined by claim 1' wherein said organic peroxide is percent stable at up to at least 1 10 C., and decomposes when heated to about l40-220 C. 

2. A polymeric mass as defined by claim 1 wherein there are employed 0.005 to 2 percent by weight of an organic peroxide, and 0.05 to 4 percent by weight of sulfur.
 3. A polymeric mass as defined by claim 2, further comprising an accelerator selected from the group consisting of mercaptobenzothiazole, tetramethyl thiuram-disulfide, and mixtures thereof. 4 A polymeric mass as defined by claim 1 wherein there is incorporated from 20 to 60 percent by weight of carbon black.
 5. A polymeric mass as defined by claim 1 wherein said organic peroxide is dicumyl peroxide, dibenzoyl peroxide, tert.-butyl perbenzoate, tert.-butyl- Alpha -cumyl peroxide, di-tert.-butyl peroxide, dibenzyl peroxide, bis-(tert.-butyl-peroxy-methyl)-durene, 2-,5-dimethyl-2-,5-peroxy-3 -hexine, 2-,5-dimethyl-2-,5-diisopropyl-peroxy-hexane, 2,2-bis-(tert.-butyl-peroxy)-butane, or isopropylidene-2,5-dimethylhexane-2,5-diperoxide.
 6. A polymeric mass as defined by claim 2 wherein said organic peroxide is dicumyl peroxide, dibenzoyl peroxide, tert.-butyl perbenzoaTe, tert.-butyl- Alpha -cumyl peroxide, di-tert.-butyl peroxide, dibenzyl peroxide, bis-(tert.-butyl peroxy-methyl)-durene, -peroxide, 2-,5-dimethyl-2-,5-peroxy-3-hexine, 2-,5-dimethyl-2-,5diisopropyl-peroxy-hexane, 2,2-bis-(tert.-butyl-peroxy)-butane, or isopropylidene-2,5-dimethylhexane-2,5-diperoxide.
 7. A polymeric mass as defined by claim 3 wherein said organic peroxide is dicumyl peroxide, dibenzoyl peroxide, tert.-butyl perbenzoate, tert.-butyl- Alpha -cumyl peroxide, di-tert.-butyl peroxide, dibenzyl peroxide, bis-(tert.-butylperoxy-methyl)-durene, peroxide, 2-,5-dimethyl2-,5-peroxy-3-hexine, 2-, 5dimethyl-2-,5-diisopropyl-peroxy-hexane, 2,2-bis-(tert. -butyl-poroxy)-butane, or isopropylidene-2,5-dimethylhexane-2,5-diperoxide.
 8. A polymeric mass as defined by claim 1, said organic peroxide being present in a concentration of 0.05-1 percent by weight, and being selected from the group consisting of 2-,5dimethyl-2, 5-peroxy-3-hexyne, 2-,5-dimethyl-2,5-diisopropylperoxy-hexane, isopropylidene-2,5-dimethyl-hexane-2,5-diperoxide, and mixtures thereof, and said sulfur being present in a concentration of 0.2-0.5 percent by weight.
 9. A polymeric mass as defined by claim 1 wherein said organic peroxide is 80 percent stable at up to at least 110* C., and decomposes when heated to about 140*-220* C. 